GEPI

Subject : Up to 2018, the main limitation to construct a possible scenario of the formation of the Galaxy in the first billions of years after the Big Bang has been an insufficiently precise dating of stars, a lack of a reliable chronology of the history of our Galaxy at that time. During that period, the formation of the Milky Way has been a complex mix of different events and processes which is difficult to comprehend. Precise stellar dating would make it possible to understand when the different events (formation of the Galactic disk, satellite galaxy accretions, etc..) that contributed to the Galactic formation occured, and how they are connected to each other. Combined with newly available spectroscopic data, the publication of the Gaia second catalogue (DR2) has provided the necessary data to revisit this problem entirely.

The proposed internship is based on the opportunity offered by these new data. The first aim is to stellar date some of the old main sequence stars that are within a couple of kpc from the Sun. The age will be inferred using Gaia parallaxes and photometry, chemical information coming from spectroscopic surveys, and comparisons with stellar isochrones using bayesian estimation. Combined with kinematical information and the knowledge of their orbital parameters, these stars should be relatively easy to characterize, giving access to a sample significantly more extended than those available up to now in the solar vicinity. The sample will also be compared with more local, but more precise, solar vicinity data. The results of this investigation will then be discussed in a larger context, and confronted to the various ideas and scenarios that have flourished since the publication of the Gaia catalogue on the age of the oldest populations of the Milky Way.